As the broadcast industry transitions to digital services, the complexity created by multiformat programming means that broadcasters need to look carefully at how they will handle closed captions (CC). What was once the relatively simple act of inserting CC data into line 21 of a National Television System Committee (NTSC) signal has now, as has every other aspect of DTV infrastructure, expanded exponentially in complexity.
FIG. 1 illustrates a digital television (DTV) screen shot 101 including closed caption text 102. Since Jul. 1, 2002, the Federal Commerce Commission (FCC) has required U.S. broadcasters to provide closed captioning on at least some of their programs transmitted on DTV channels. No distinction is made in the FCC rules between Digital Television Closed Captioning (DTVCC) for high definition or for standard definition programming; requirements are the same for both.
According to rules of the Federal Commerce Commission (FCC), all DTV set-top boxes are now to include a caption decoder complying with CEA-708-B of the Consumer Electronics Association. Decoders must be able to decode the six standard services in CEA-708-B and allow users to choose at least one for display. Set-top boxes that have an analog NTSC output are expected to insert CEA-608-B caption data carried in the DTV signal into line 21 of the NTSC video output in order to comply with FCC rules.
FIG. 2 illustrates how closed captioning is carried in the video user bits of the MPEG-2 bitstream. The CEA-608-B standard defines the coding of data, including captioning. Captions generated in accordance with CEA-608-B are commonly referred to as “608” captions. The 608 standard allows for up to four caption services per program and provides the familiar limited range of white characters (color is allowed but rarely used) within a box-like black background field.
The CEA-708-B (“708”) standard defines the coding of DTVCC as they are delivered in an Advanced Television Systems Committee (ATSC) emission bitstream as specified by ATSC. This standard is applicable equally to high definition and standard definition video formats and to both satellite broadcasting and cable distribution.
DTVCC captions have greatly enhanced formatting and display capabilities compared to 608 captions, with up to 63 services per program (only 16 can be announced in Program and System Information Protocol (PSIP)), eight independently controlled display windows, and an extended range of characters and multiple fonts, sizes and background and character colors and edges. The 708 standard specifies how caption information is to be coded and processed, minimum implementation recommendations for Digital Television (DTV) closed caption decoders, and recommended practices for caption encoder and decoder manufacturers. The 708 standard also requires that decoders give users control over caption font, color, size and location that may override the parameters as transmitted.
In addition to the actual 708 caption data required for use by a DTV set-top box or integrated receiver to display DTV captions, the 708 standard allows for carriage of equivalent data for a subset of the captions coded in the 608 format for NTSC-based receivers. This legacy 608 data, also known as the “608 compatibility bytes,” is required for use by DTV set-top boxes so that line 21 data can be inserted in an analog composite video output (if provided) to feed a legacy NTSC TV set. Some DTV receivers may use the 608 data when 708 data is not available for providing closed captioning on the DTV display, but this is not mandatory and is not implemented in some receivers.
The 708 standard defines the Caption Distribution Packet (CDP) consisting of a specific sequence of bytes that can hold the actual 708 DTV caption data, the 608 caption data, caption service information, and optional time code for synchronization (not needed with transport methods where the CDP is directly associated with video frames). The CDP is the basic unit of data that is transported through the professional portion of a DTV caption distribution chain.
Caption service information comprises the caption service descriptor (CSD) as defined in the ATSC PSIP standard. It is used in the ATSC transport stream EIT (event information table) and PMT (program map table) to announce the presence and format of captions being carried.
It is possible for captions to be encoded in the 708 DTVCC format by conversion from 608 legacy captions already encoded in line 21 of an analog NTSC video feed. This method is used when the program was created primarily for NTSC transmission but is being encoded for DTV transmission, either as a standard definition (SD) program or with up-conversion to a high definition (HD) program. For new programming this is expected to be an interim solution until native 708 caption authoring is generally adopted, but may persist indefinitely for legacy material that is already captioned.